Field of the Invention
The invention lies in the semiconductor technology field. More specifically, the invention relates to a method of producing a II-VI semiconductor component, in which an active layer sequence having at least one II-VI semiconductor layer containing Se and/or S is applied on a substrate. It relates, in particular, to a method of producing laser diodes having a layer sequence exhibiting laser activity and consisting essentially of ZnMgSSe or BeMgznSe, in particular on a GaAs, Si or Ge substrate by means of molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD).
The use of II-VI laser diodes based on ZnMgSSe or BeMgznSe has been unsuccessful in the state of the art because of the short lifetime which it has to date been possible to achieve for these components. The cause attributed to the deterioration, which follows a diffusion-limiting mechanism, are non-radiating areas, referred to as xe2x80x9cdark spotsxe2x80x9d (DS) or xe2x80x9cdark line defectsxe2x80x9d (DLD), which propagate and proliferate during operation of the laser diodes. On account of their structure, DSs and DLDs are identified as dislocation loops and dislocation dipoles in or in the vicinity of the active region. Their origin lies predominantly in extended crystal defects, such as, for example, dislocations or stacking faults, which for the most part occur at the interface between the II-VI layer sequence and the III-V substrate (cf. L. H. Kuo et al., Generation of Degradation Defects, Stacking Faults and Misfit Dislocations in ZnSe-Based Films Grown on GaAs, J. Vac. Sci. Technol. B, 13(4) (1995), 1694).
The nucleation of these multidimensional lattice defects may be due to a tendency toward chemical reaction between selenium atoms, or sulfur atoms, and the GaAs surface. Both of these chalcogens form strong bonds to III-V semiconductors, in particular to those containing Ga and In, such as GaAs, InAs or InGaAs. The resulting reaction productsxe2x80x94for example Ga2Se3 or Ga2S3 are proposedxe2x80x94for numerous seeds at the substrate surface for the creation of stacking defects. This seed formation can occur even with small amounts of sulfur or selenium in the background pressure of the epitaxy reactor. This undesired contamination of the substrate surface with Se or S can take place by evaporation of these elements from hot filaments or furnace diaphragms; it is therefore extremely difficult to prevent this in a II-VI epitaxy reactor.
In order to stop the incorporation of stacking defects at the start of growing ZnSe on GaAs, various MBE techniques have been developed in which the reaction between Se and Ga is prevented. In this case, before the II-VI semiconductor layer is grown, the GaAs substrate is passivated, for example with Zn or Te, and this makes direct contact to Se atoms with the GaAs surface more difficult. To that end, the substrate is exposed to a Zn beam at low temperatures of about 230xc2x0 C. inside the II-VI growth chamber, without the activation energy needed for the reaction between Se and Ga to take place being provided. For kinetic reasons, during the growth of ZnSe at such low temperaturesxe2x80x94ZnSe is typically produced at between 270xc2x0 C. and 320xc2x0 C.xe2x80x94a transition to three-dimensional growth (island growth) takes place. Under these conditions, the coalescence of growth islands can lead to the incorporation of defects. The use of island growth can be circumvented using an MEE (migration enhanced epitaxy) process. During MEE growth, the crystal surface is presented alternately with Zn and Se, the atoms of a monolayer being provided, between each cycle, in spite of short diffusion lengths, with the time to take occupancy of favorable sites on the surface. Using this process, it has been possible to reduce the defect density in ZnSe or ZnSSe on GaAs to below 105 cmxe2x88x922 (cf. J. M. Gaines et al., Structural Properties of ZnSe Films Grown by Migration enhanced Epitaxy, J. Appl. Phys. 73(6) (1993) 2835, and C. C. Chu et al., Reduction of Structural Defects in II-VI Blue-Green Laser Diodes, Appl. Phys. Lett. 69(5) (1996), 602).
Passivation with Te atoms provides a possible alternative to the Zn treatment (Zn-MEE). The chemical reactivity of Te with GaAs is significantly less than that of Se and Sxe2x80x94Te/GaAs interfaces ought therefore to exist more stably in the crystal structure of the semiconductor matrix than Se/GaAs or S/GaAs. Under experimental conditions, however, poor adhesion of Te has been observed, and it was not possible to demonstrate any clear reduction of the defect density.
For an industrial process step in the production of II-VI semiconductor lasers, the proposed methods of preventing extended defects at the start of growing ZnSe on GaAs exhibit excessively poor reproducibility. A disadvantage with Zn preparation is therefore that the resultant Zn-As interlayer does not give it a defined surface, and dislocations can sometimes nucleate. Further, Se atoms from hot surfaces in the MBE reactor still continue to impair the passivation process. Selenium or sulfur in the background pressure have a similar effect on the Te passivation owing to the exchange reaction which takes place, in which Te is replaced by Se, and the concomitant low adhesion coefficient of Te, the Te passivation is less effective at protecting the GaAs substrate.
The object of the invention is to provide a method of producing a II-VI semiconducting component which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which is simple in its implementation and with which the creation of stacking defects and dislocations at the junction between the substrate and the II-VI semiconductor material is prevented.
With the above and other objects in view there is provided, in accordance with the invention, a method of producing a semiconductor component with an active layer sequence with one or more II-VI semiconductor layers. The method comprises the following steps:
providing a substrate;
epitaxially growing an Se-free II-VI interlayer based on BeTe on the substrate in a substantially Se-free and S-free first epitaxy chamber; and
epitaxially growing an active layer sequence with at least one II-VI semiconductor layer containing an element selected from the group consisting of Se and S on the Se-free II-VI semiconductor layer.
With the novel method according to the invention, it is advantageously possible to produce BeTe interlayers with high quality on a substrate, for example GaAs, using MBE. In this case, the BeTe layer acts as a buffer between a II-VI semiconductor layer containing Se or S, for example ZnMgSSe or BeMgZnSe, in such a way that no stacking defects or new dislocations are created at the interface with the substrate and spread through the layers above.
In accordance with an added mode of the invention, the inter-layer consists of BexMgyZn1xe2x88x92xxe2x88x92yTe, BexZnyCd1xe2x88x92xxe2x88x92yTe, BexZnyMn1xe2x88x92xxe2x88x92yTe, or BexMnyCd1xe2x88x92xxe2x88x92yTe.
In accordance with an additional feature of the invention, the substrate is a III-V semiconductor material, preferably GaAs, InAs, or InGaAs.
According to the invention, in particular, before the growth of an optoelectronic or electronic component made of II-VI semiconductor material, in particular BeMgZnSe, ZnMgSSe, MgZnCdSe, MgZnCdS or BeMgZnS, in a first essentially Se-free epitaxial reactor, a BeTe interlayer is deposited on the substrate crystal, which consists in particular of GaAS or InAs.
Using a BeTe interlayer to improve the start of the MBE growth of a selenide, for example BeMgZnSe or ZnMgSSe, on GaAs has already been described in international PCT publication WO 97/18592. A disadvantage with the process described therein is, however, that the process is only conditionally reproducible under the normal conditions of II-VI epitaxy, in particular the high proportion of Se and S in the base pressure of the reactor. Further, the electrical transport properties are impaired with the layer thicknesses proposed there for the buffer layer, since BeTe represents a barrier to electrons which becomes more and more difficult to tunnel through as its thickness increases.
In accordance with another feature of the invention, a thickness of the interlayer is between 0.5 to 100 monolayers.
In accordance with a further feature of the invention, a smooth buffer layer is applied on the substrate prior to the step of epitaxially growing the Se-free II-VI interlayer.
In accordance with again an added feature of the invention, the smooth buffer layer is formed, depending on the semiconductor material of the substrate, from GaAs, InAs, InGaAs, InP, GaP, GaSb, GaN, or mixed crystals formed thereof, from Ge, Si, SiGe, SiC, SixC1xe2x88x92x, or mixed crystals formed thereof (0xe2x89xa6xxe2x89xa61), from ZnO, ZnSe, CdTe, CdZnTe, or mixed crystals formed thereof, or from Al2O3. The smooth buffer layer may be an undoped layer, an n-type conductive doped layer, or a p-type conductive doped layer.
In accordance with again an additional feature of the invention, the interlayer is produced on the smooth upper layer in the same epitaxy chamber in which the buffer layer is produced.
In accordance with again a further feature of the invention, an As-rich surface is grown prior to growing the interlayer on the buffer layer.
In accordance with a concomitant feature of the invention, a matching layer is grown on the interlayer before the active layer sequence is grown.
By virtue of the process according to the invention, the density of extended crystal defects in the II-VI semiconductor component is lowered reproducibly. As a result of this, especially in the case of optoelectronic components, long-term stability and emission characteristics are improved significantly. The application according to the invention of a thin BeTe interlayer on, for example, a GaAs substrate and of a II-VI semiconductor layer, for example made of BeMgZnSe or ZnMgSSe, prevents selenium or sulfur from being able to reach the GaAs surface, which prevents the creation of stacking defects and dislocations at the junction between the II-VI semiconductor and the III-V semiconductor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of producing a II-VI semiconductor component, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.